Particle therapy system

ABSTRACT

A particle therapy system is capable of reducing an increase in treatment time caused by the initialization operation of magnets in the execution of the scanning irradiation method successively changing the energy level of a beam extracted from an accelerator. An irradiation control apparatus has a scheme that calculates setting vales of excitation current for bending magnets for a transport system on every irradiation condition (energy condition), and sets appropriate excitation current values according to the irradiation sequence. The irradiation control apparatus  35  prestores in a current supply control table  1  reference current values determined corresponding to energy levels of the ion beam, prestores in current supply compensation value tables  1, 2  compensation current values determined corresponding to energy levels of the ion beam and numbers of times of changing the energy level, and calculates the excitation current value of the magnets by using the values prestored in the tables.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a particle therapy system for treatingtumors such as cancers by irradiating the tumors with a proton beam or aheavy ion beam such as a carbon ion beam.

2. Description of the Related Art

A particle therapy, which irradiates a target volume (tumor volume) withan ion beam such as a proton beam or a carbon ion beam, is well known asone of the cancer treatment methods. When an ion beam (proton beam,carbon ion beam, etc.) with high energy enters a material, the ion beamloses much of its energy at the end of its propagation path (range). Theparticle therapy takes advantage of such a property of the ion beam andapplies the ion beam to the patient so as to make the beam lose much ofits energy in cancer cells. In the particle therapy, a dose distributionconformal to the shape of the target volume is formed by adjusting thespatial broadening (spatial distribution) and the energy of the ion beam

A particle therapy system used for the particle therapy comprises an ionsource for generating ions, an accelerator for accelerating the ionsgenerated by the ion source and thereby forming an ion beam, a beamtransport system for transporting the ion beam extracted from theaccelerator, and an irradiation device for irradiating the target volumewith the ion beam according to a desired dose distribution.

The accelerator used for the particle therapy system can be asynchrotron or a cyclotron, for example. The function of acceleratinginjected ions to a prescribed energy level and outputting theaccelerated ions as an ion beam is common to the synchrotron and thecyclotron.

The ion beam extracted from the accelerator is transported by the beamtransport system to the irradiation device. The beam transport system isequipped with bending electromagnets (hereinafter referred to as“magnet” or “magnets”) for changing the propagation direction of the ionbeam, steering electromagnets (hereinafter referred to as “magnet” or“magnets”) for the fine adjustment of the beam propagation direction,and quadrupole electromagnets (hereinafter referred to as “magnet” or“magnets”) for giving convergence/divergence effects to the ion beam. Byproperly adjusting the levels of excitation of these magnets, a beam inan appropriate size and at an appropriate position can be transported tothe irradiation device.

There are cases where the beam transport system and the irradiationdevice are mounted on a rotary gantry in order to irradiate the targetvolume with beams from multiple directions. The beam transport system ofthe particle therapy system having the rotary gantry can be roughlydivided into a rotary beam transport system which is mounted on therotary gantry and a fixed beam transport system which is mounted/fixedon the building.

The irradiation device forms an ion beam irradiation field that isconformal to the shape of the target volume. The irradiation field canbe formed by two types of methods: a scatterer irradiation method and ascanning irradiation method. With the technological progress, themainstream is shifting toward the scanning irradiation method capable ofhigh-accuracy irradiation. An irradiation device employing the scanningirradiation method is equipped with two scanning electromagnets(hereinafter referred to as “magnet” or “magnets”) for scanning thebeam. In order to irradiate exclusively the target volume, the beam inthe irradiation device is scanned by these scanning magnets within aspecified area orthogonal to the beam propagation direction. Bysuccessively changing the energy of the beam extracted from theaccelerator, the reachable depth of the beam can be changed and theirradiation field conformal to the shape of the target volume can beformed.

Further, in the particle therapy, the reproducibility of the beamirradiation position is generally enhanced by performing initializationoperation on the magnets to maintain the beam irradiation positionaccuracy at a high level.

A method for enhancing the accuracy and the reproducibility of the beamirradiation position has been described in JP-2005-296162-A. In thismethod, the energy level of the beam extracted from the accelerator ischanged successively in order to form the irradiation field conformal tothe shape of the target volume. To avoid ill effect of the hysteresis ofthe scanning magnets, a conversion table regarding conversion betweenbeam position data detected by a beam position monitor and presetcurrent values of the scanning magnets is stored in a storage device andthe current values (electric current values) of the scanning magnets areset by using the stored conversion table according to beam position datadetermined based on treatment plan data.

JP-8-298200-A has described a method for uniformizing the remanentmagnetization of the synchrotron magnets on each switching of the energylevel in the operation successively changing the energy level of the ionbeam. In this method, an initialization operation of temporarilyincreasing the excitation current value of each magnet (reexcitation) toa current value for the initialization (without shifting to the beamdeceleration process immediately after the completion of the beamextraction at each energy level) and then demagnetizing each magnet isperformed also on the magnets of the extraction beam transport system inthe same way as the synchrotron magnets.

SUMMARY OF THE INVENTION

As mentioned above, the scanning irradiation method successively changesthe energy level of the beam extracted from the accelerator in order toform the irradiation field conformal to the shape of the target volume.To carry out the treatment irradiation with high beam irradiationposition accuracy in the operation successively changing the energylevel, it is desirable to perform the initialization operation on thesynchrotron (accelerator) magnets and the transport system magnets oneach switching of the energy level.

The conventional technique of JP-2005-296162-A is a proposal of a methodfor avoiding the ill effect of the hysteresis of the scanning magnetswhen the beam is scanned in one layer, and thus is not a technique forcompensating for the change in the remanent magnetization of theaccelerator magnets and the transport system magnets on each switchingof the energy level.

The conventional technique of JP-8-298200-A performs the initializationoperation not only on the synchrotron (accelerator) magnets but also onthe transport system magnets on each switching of the energy level bytemporarily increasing the excitation current value of each magnet tothe initialization current value (reexcitation) and then demagnetizingeach magnet.

In general, each of the synchrotron magnets operates on a pattern powersupply and carries out pattern operation of successively increasing anddecreasing the magnetic field intensity in the order of injection,acceleration, extraction and deceleration at each energy level. On theother hand, each of the transport system magnets generally operates on aDC power supply and carries out operation of changing its excitationcurrent value (set at a constant value corresponding to the energy levelof the ion beam) stepwise on each switching of the energy level.

If the initialization operation of temporarily increasing the excitationcurrent value to the initialization current value (reexcitation) andthen dropping the excitation current value (demagnetization) isperformed on the transport system magnets (operating on DC powersupplies as mentioned above) on each switching of the energy levelaccording to the idea of JP-8-298200-A, the following problem occurs:Since the responsiveness of DC power supplies is three to four timesslower than that of pattern power supplies, the time necessary for theswitching of the energy level increases considerably and the treatmentirradiation time can be doubled depending on the conditions of theirradiation. If pattern power supplies are employed for the transportsystem magnets, the increase in the treatment irradiation time can beavoided differently from the case employing DC power supplies. However,this leads to an increase in the power supply cost.

It is therefore the primary object of the present invention to provide aparticle therapy system capable of reducing the increase in thetreatment time caused by the initialization operation of the magnets inthe execution of the scanning irradiation method successively changingthe energy level of the beam extracted from the accelerator.

In order to achieve the above object, a particle therapy system inaccordance with the present invention comprises an irradiation controlapparatus which controls the excitation current value of at least onemagnet installed in the accelerator or the beam transport system so asto increase or decrease the excitation current value stepwise based onthe energy level of the extracted ion beam and the number of times ofstepwise changing of the energy level of the extracted ion beam.Specifically, the irradiation control apparatus prestores referencecurrent values determined corresponding to energy levels of the chargedparticle beam (ion beam) and compensation current values determinedcorresponding to energy levels of the ion beam and numbers of times ofchanging the energy level, and calculates the excitation current valueof the magnet by using the reference current value and the compensationcurrent value. With this configuration, excitation current valuesavoiding the influence of the energy level of the ion beam and thenumber of times of changing the energy level can be set to magnetsoperating on DC power supplies (especially, bending magnets in thetransport system). Consequently, treatment irradiation control with highaccuracy of the beam irradiation position becomes possible withoutcausing an increase in the treatment time or the power supply cost.

According to the present invention, it is possible to provide a particletherapy system capable of reducing the increase in the treatment timecaused by the initialization operation of the magnets in the executionof the scanning irradiation method successively changing the energylevel of the beam extracted from the accelerator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing the overall configuration ofa particle therapy system in accordance with an embodiment of thepresent invention.

FIG. 2 is a schematic diagram for explaining the scanning irradiationmethod in which the target volume is segmented into layers and thetarget volume is irradiated with an even dose by successivelyirradiating each layer with a particle beam while scanning the particlebeam in lateral directions with scanning magnets.

FIG. 3 is a graph showing excitation current patterns for synchrotronmagnets and transport system magnets during the scan irradiationoperation.

FIG. 4 shows a current supply control table which is used for magnetexcitation current control.

FIG. 5 shows a current supply compensation value table which is used forthe magnet excitation current control.

FIG. 6 shows another current supply compensation value table which isused for the magnet excitation current control.

FIG. 7 is a graph showing beam position deviation at the irradiationposition caused by magnetic field hysteresis of transport system bendingmagnets when energy scan operation is performed.

FIG. 8 is a graph showing the beam position deviation at the irradiationposition in cases where the energy scan operation is performed indifferent energy ranges.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, a description will be given in detail ofa preferred embodiment in accordance with the present invention.

FIG. 1 is a schematic block diagram showing the overall configuration ofa particle therapy system in accordance with an embodiment of thepresent invention. In this embodiment, the particle therapy system isconfigured as explained below.

As shown in FIG. 1, the particle therapy system of this embodimentcomprises an ion beam generating device (particle beam generatingdevice) 1, a high-energy beam transport system 2 connected downstream ofthe ion beam generating device 1, and a rotary gantry 4 having a gantrybeam transport system 3.

The ion beam generating device 1 includes a synchrotron 6 and a linearaccelerator 5 having an ion source. The synchrotron 6 includes anextraction high-frequency power application device 7 and an accelerationhigh-frequency power application device 8. The extraction high-frequencypower application device 7 (high-frequency power application device forthe extraction of the ion beam from the synchrotron 6) is formed byconnecting a high-frequency power application electrode 9 placed in theorbit of the synchrotron 6 to a high-frequency power supply 10 via anON/OFF switch 11. The linear accelerator 5 generates an ion beam(particle beam) by accelerating ions (protons, carbon ions, etc.)generated in the ion source and injects the ion beam into thesynchrotron 6. The ion beam circulates in the synchrotron 6 and isaccelerated up to a preset energy level when necessary energy is givenby acceleration energy generated by the acceleration high-frequencypower application device 8. The energy level of the ion beam isdetermined to suit the depth of the target volume in the patient's bodyfrom the body surface (irradiation depth) measured in the beamirradiation direction. When necessary energy has been given to the ionbeam circulating in the synchrotron 6, extraction high-frequency power(high-frequency electric power used for the extraction of the ion beam)is supplied from the high-frequency power supply 10 to thehigh-frequency power application electrode 9 via the closed switch 11and is applied to the ion beam by the electrode 9. The ion beamcirculating within the stability limit is shifted to the outside of thestability limit by the application of the high-frequency power, by whichthe ion beam is extracted from the synchrotron 6 via an extractiondeflector 12. For the extraction of the ion beam, electric currentssupplied to quadrupole magnets 13 and bending magnets 14 of thesynchrotron 6 are kept at preset values (electric current set values)and the stability limit is also kept substantially at a constant level.The extraction of the ion beam from the synchrotron 6 is ended bystopping the application of the high-frequency power to thehigh-frequency power application electrode 9 by opening the switch 11.

The ion beam (hereinafter referred to simply as a “beam”) extracted fromthe synchrotron 6 is transported downstream (toward an irradiationnozzle 21) by the high-energy beam transport system 2. The high-energybeam transport system 2 includes a bending magnet 15, quadrupole magnets16 and steering magnets 17. The beam supplied to the high-energy beamtransport system 2 is lead to the gantry beam transport system 3 via abending magnet 15. The gantry beam transport system 3 attached to therotary gantry 4 includes quadrupole magnets 18, steering magnets 19, andbending magnets 20A, 20B and 20C. The beam lead to the gantry beamtransport system 3 is transported to the irradiation nozzle 21(irradiation device) via these magnets. The irradiation nozzle 21,employing the scanning irradiation method, has been mounted on a rotarydrum of the rotary gantry 4. A profile monitor 22, scanning magnets 23,a dose monitor 24, and a spot position monitor 25 are arranged insidethe irradiation nozzle 21. The beam entering the irradiation nozzle 21passes through the profile monitor 22 placed on the beam path, getsdeflected by two scanning magnets 23, passes through the dose monitor 24and the spot position monitor 25, and then irradiates the target volumeinside the body of the patient 30.

The particle therapy system of this embodiment comprises an irradiationcontrol apparatus 35 for controlling the irradiation of the beam. Asshown in FIG. 1, the irradiation control apparatus 35 includes a centralcontrol unit 50, a nozzle controller 51, an accelerator controller 52,and a transport system controller 53. The central control unit 50 isconnected to the nozzle controller 51, the accelerator controller 52 andthe transport system controller 53. The central control unit 50 isconnected also to a treatment planning apparatus 40 to receive treatmentplan data sent from the treatment planning apparatus 40. The nozzlecontroller 51 controls the profile monitor 22, the scanning magnets 23,the dose monitor 24 and the spot position monitor 25. The acceleratorcontroller 52 controls the devices constituting the synchrotron 6. Thetransport system controller 53 controls the devices constituting thehigh-energy beam transport system 2 and the gantry beam transport system3. Further, a human interface terminal (HMI terminal) 31 is connected tothe central control unit 50.

The central control unit 50 loads the aforementioned treatment plan dataregarding the patient (who is going to receive the treatment) from thetreatment planning apparatus 40. As mentioned above, the irradiationdepth corresponds to the energy level of the beam. The energy level ofthe beam corresponds to the control pattern of the excitation currentssupplied to the magnets of the synchrotron 6, the high-energy beamtransport system 2 and the gantry beam transport system 3. Specifically,an electric power supply control table has been prestored in the centralcontrol unit 50. For example, values or patterns of excitation power tobe supplied to the quadrupole magnets 13 and the bending magnets 14 ofthe ion beam generating device 1 including the synchrotron 6 and to thequadrupole magnets 16 and 18, the steering magnets 17 and 19 and thebending magnets 15, 20A, 20B and 20C of the beam transport systems 2 and3 have been preset for each energy level (150 MeV, 145 MeV, 140 MeV,etc.).

Next, the beam irradiation method employed by the particle therapysystem of this embodiment will be explained referring to FIG. 2. Thetarget volume is segmented into laminar regions (layers) as indicatedwith the reference characters 1, 2, 3 and 4 in FIG. 2. In this case, itis possible to irradiate each layer with a beam of a constant energylevel. The layer 1 is situated at the deepest position in the beampropagation direction and the layers 2, 3 and 4 gradually get shallowerin this order. Each layer is irradiated by use of the two scanningmagnets 23 so that the entire layer is scanned by the beam. The centralcontrol unit 50 sends the irradiation pattern which has been set by thetreatment planning apparatus 40 to the nozzle controller 51. The nozzlecontroller 51 controls the irradiation nozzle 21 so as to scan the beamaccording to the data (irradiation pattern) and irradiate each layerwith the preset dose. When the irradiation of a layer is finished, thenozzle controller 51 sends an irradiation completion signal regardingthe layer to the central control unit 50. The central control unit 50receiving the irradiation completion signal sends a beam energyalteration signal to the accelerator controller 52 and the transportsystem controller 53. In response to the beam energy alteration signal,the accelerator controller 52 and the transport system controller 53 seta magnet excitation current set value corresponding to the next energylevel to a power supply unit of each magnet, by which beam extractionpreparation for the next energy level is completed. When the beamextraction preparation is completed, the central control unit 50 sendsan extraction preparation completion signal to the nozzle controller 51.In response to the extraction preparation completion signal, the nozzlecontroller 51 starts the irradiation of the next layer. As explainedabove, the beam is scanned in a three-dimensional pattern by scanningthe beam laterally (in the lateral directions) by using the scanningmagnets and successively changing the beam energy in the depthdirection. The above irradiation method is the one called “scanningirradiation method”. The scanning irradiation method is an irradiationmethod capable of giving the dose exclusively to the target volume andpreventing the irradiation of normal tissue around the target volume.Incidentally, the operation successively changing the energy(specifically, increasing/decreasing stepwise the energy level of theion beam extracted from the accelerator) is called “energy scanoperation”.

Next, the operation of the irradiation control apparatus 35 for theenergy scan operation, as a characteristic feature of this embodiment,will be explained below.

The central control unit 50 receives prescription data regarding thepatient (who is going to receive the treatment) from the treatmentplanning apparatus 40. The prescription data includes information on thetreatment room, the angles of the rotary gantry 4, the beam energy(energy of the irradiating beam), the beam scan ranges, the irradiationamount, etc. Based on the prescription data, the central control unit 50generates control command data (control command information) forcontrolling the magnets arranged along the beam path inside the ion beamgenerating device 1 and the beam transport systems 2 and 3. The controlcommand data is generated by use of tables specifying magnet excitationlevels for each energy level and is outputted to the power supply unitsof the magnets. Among the power supply units, those of the magnets ofthe synchrotron 6 (hereinafter referred to as “synchrotron magnets”) areimplemented by using pattern power supplies having high response speeds,while those of the magnets of the high-energy beam transport system 2and the gantry beam transport system 3 (hereinafter referred to as“transport system magnets”) are implemented by using low-priced DC powersupplies.

Here, the control of the excitation currents supplied to the synchrotronmagnets and the transport system magnets during the energy scanoperation in this embodiment will be explained. FIG. 3 is a graphshowing the excitation current patterns for the synchrotron magnets andthe transport system magnets during the energy scan operation in whichthe energy of the extracted ion beam is decreased stepwise.

In the particle therapy according to this embodiment, initializationoperations of the magnets (indicated with reference characters 100A and100B in FIG. 3) are conducted first in order to carry out the beamirradiation with high reproducibility of the beam position. Themagnetization of the magnets arranged on the beam path can beuniformized by the initialization operation, by which variations in themagnetization can be suppressed and the reproducibility of the beamirradiation position can be increased. Subsequently, the synchrotronmagnets (operating on the pattern power supplies) are operated inpattern operation of increasing and decreasing the magnetic fieldintensity as indicated with the reference character 101A in FIG. 3 evenafter the initialization operation. The excitation current value of eachsynchrotron magnet in the ion beam extraction period is controlled sothat the value decreases stepwise and in stages to suit the energy ofthe extracted ion beam. On the other hand, each of the transport systemmagnets (operating on the DC power supplies) after the initializationoperation is set at a fixed excitation current value to suit the energyof the extracted ion beam as indicated with the reference character 101Bin FIG. 3. The excitation current value of each transport system magnetis changed stepwise corresponding to the change in the energy level. Ingeneral, the time necessary for changing the energy level isapproximately 1-10 seconds (corresponding to one operation cycle of thesynchrotron) although the time varies depending on the conditions of theirradiation.

Further, in the energy scan operation according to this embodiment, theinitialization operation is performed on the synchrotron magnets onevery switching of the energy level as indicated with the referencecharacter 102A in FIG. 3 in order to carry out the treatment irradiationwith high positional accuracy. In this initialization operation 102A,the excitation current value is temporarily increased to a current value(maximum current value) for the initialization (reexcitation) withoutimmediately shifting to the deceleration process after the completion ofthe beam extraction at each energy level, and then the magnets aredemagnetized. Therefore, the initialization operation 102A is differentfrom the aforementioned initialization operations 100A and 100B.

It is possible to perform the initialization operation on everyswitching of the energy level (temporarily increasing the excitationcurrent value to the initialization current value (reexcitation) andthen demagnetizing each magnet) also on the transport system magnets(operating on the DC power supplies) in the same way as the synchrotronmagnets. However, the responsiveness of DC power supplies is three tofour times slower than that of pattern power supplies and thusperforming the initialization operation by using the DC power suppliesconsiderably increases the time necessary for the switching of theenergy level. As a result, the treatment irradiation time can be doubleddepending on the irradiation conditions. The increase in the treatmentirradiation time can of course be avoided by employing pattern powersupplies also for the transport system magnets; however, this leads toan increase in the power supply cost.

In this embodiment designed in consideration of the above problems,excitation current control for making it possible to perform thetreatment irradiation without the initialization operation of thetransport system magnets is carried out while employing conventional DCpower supplies for the transport system magnets, especially for thetransport systems' bending magnets 15, 20A, 20B and 20C having themaximum effect on the positional accuracy of the beam irradiation. Theoutline of the excitation current control is as follows: The excitationcurrent value of each magnet is calculated by use of a reference currentvalue determined according to the energy (energy level) of the extractedbeam and a correction value (compensation value) determined according tothe number of times of the change of the energy level to reach theintended energy level. Each magnet is controlled according to thecalculated excitation current value. The details of the excitationcurrent control will be explained below.

FIG. 4 shows a current supply control table 1 (first data table) usedfor the magnet excitation current control of the transport systems. FIG.5 shows a current supply compensation value table 1 (second data table)used for the magnet excitation current control. FIG. 6 shows a currentsupply compensation value table 2 (third data table) used for the magnetexcitation current control. The magnet excitation level for each energylevel (i.e., the value of the excitation current supplied to each magnetfor the beam irradiation at each energy level) is calculated by usingthe three data tables 1, 2 and 3.

The current supply control table 1 stores reference magnet excitationcurrent values (i.e., reference values of the excitation currentsupplied to each magnet). Specifically, the current supply control table1 stores the value of the excitation current to be supplied to eachmagnet when the beam irradiation at a certain energy level is conductedwithout considering the remanent magnetization, that is, whenmonoenergetic operation is performed without executing the energy scanoperation. This table, for registering excitation current valuesnecessary for realizing bending magnet excitation levels capable ofproperly deflecting the beam during the monoenergetic operation, isdetermined during the beam commissioning.

The current supply compensation value tables 2 and 3 are tables used forthe fine adjustment of the control command data stored in the currentsupply control table 1. Specifically, current set values (electriccurrent set values) for compensating for a change in the magnetic field(remanent magnetization) due to the hysteresis caused by the energy scanoperation are registered in the current supply compensation value tables2 and 3.

The details of the current supply compensation value tables 2 and 3 willbe explained below.

FIG. 7 is a graph showing the beam position deviation at the irradiationposition caused by the magnetic field hysteresis of the transport systembending magnets when the energy scan operation is performed. The graphwith white circles was obtained by starting the energy scan operation at225 MeV (irradiation start energy) and changing the energy level atintervals of 5 MeV. The graph with squares was obtained by starting theenergy scan operation at 225 MeV (irradiation start energy) and changingthe energy level at intervals of 15 MeV. Each number (#1, #2, etc.)represents the number of times of the energy scan. It can be seen thatthe beam position deviation changes depending on the number of times ofthe energy scan. In other words, even if the irradiation is performed atthe same energy level, the beam position deviation varies depending onthe number of times of the energy scan executed to reach the intendedenergy level.

FIG. 8 is a graph showing the beam position deviation at the irradiationposition in cases where the energy scan operation is performed indifferent energy ranges. Specifically, the beam energy was decreased sixtimes in a 200-230 MeV range and in a 170-200 MeV range. It can be seenfrom the graph that the beam position deviation varies depending on theenergy range of the energy scan operation even if the number of times ofthe energy scan is the same.

As above, the change in the magnetic field caused by the energy scanoperation is determined by the target energy level and the number ofsteps of the energy scan executed to reach the energy level. Thus, thechange in the magnetic field due to the hysteresis caused by the energyscan is compensated for by using a table describing a compensation valueratio for each energy scan step (number of times of changing the energy)as the current supply compensation value table 1 and a table describingthe reference value of the compensation (basic compensation value)corresponding to each energy level as the current supply compensationvalue table 2. By compensating for the change in the magnetic field asdescribed above, it becomes possible to omit the initializationoperation of the transport system magnets. Therefore, even when theirradiation is performed by changing the energy level of the extractedion beam, the time necessary for the switching of the energy level isshortened. Consequently, the treatment time can be reduced compared tothe conventional technology. Incidentally, the current supplycompensation value tables 2 and 3 are determined during the beamcommissioning by conducting a learning process by actually using the ionbeam.

The excitation current compensation according to this embodiment iseffective when the energy level is changed in one direction (increasedor decreased). In other words, the excitation current compensation ofthis embodiment is effective in irradiation operation in which theenergy of the extracted ion beam is increased stepwise or decreasedstepwise after the synchrotron has performed the initializationoperation. Incidentally, “changing the energy of the extracted ion beamstepwise” can mean not only the method changing the energy at fixedintervals but also other methods changing the energy by desiredincrements/decrements depending on the irradiation conditions as will beexplained later.

Next, the excitation current value compensation in this embodiment willbe explained concretely. The following explanation will be given aboutthe excitation current value of the bending magnet 20A of the beamtransport system (gantry beam transport system 3) in a case where theenergy scan operation is performed by decreasing stepwise the energy ofthe extracted ion beam in four steps of 220 MeV, 215 MeV, 210 MeV and200 MeV.

First, the magnet excitation current for the first energy level of 220MeV will be explained. According to the magnet current supply controltable 1, the reference magnet excitation current value for the bendingmagnet 20A for the energy level 220 MeV equals 410 A. Since it is thefirst step of the energy scan operation, the compensation value ratioequals 0% according to the current supply compensation value table 1.While the basic compensation value for the magnet excitation currentvalue for the energy level 220 MeV equals −2.4 A according to thecurrent supply compensation value table 2, the compensation value equals0 A since the compensation value ratio is 0%. Thus, the excitationcurrent value is determined as 410 A+0 A=410 A.

Next, the magnet excitation current for the second step (215 MeV) of theenergy scan will be explained. The reference excitation current valuefor the energy level 215 MeV equals 405 A according to the currentsupply control table 1. Since it is the second step of the energy scanoperation, the current supply compensation value ratio equals 10%according to the current supply compensation value table 1. Since thebasic compensation value for the energy level 215 MeV is −2.0 Aaccording to the current supply compensation value table 2, thecompensation value equals −0.2 A. Thus, the excitation current value isdetermined as 405 A−0.2 A=404.8 A.

Next, the magnet excitation current for the third step (210 MeV) of theenergy scan will be explained. The reference excitation current valuefor the energy level 210 MeV equals 400 A according to the currentsupply control table 1. Since it is the third step of the energy scanoperation, the current supply compensation value ratio equals 20%according to the current supply compensation value table 1. Since thebasic compensation value for the energy level 210 MeV is −2.0 Aaccording to the current supply compensation value table 2, thecompensation value equals −0.4 A. Thus, the excitation current value isdetermined as 400 A−0.4 A=399.6 A.

Finally, the magnet excitation current for the fourth step (200 MeV) ofthe energy scan will be explained. The reference excitation currentvalue for the energy level 200 MeV equals 390 A according to the currentsupply control table 1. Since it is the fourth step of the energy scanoperation, the current supply compensation value ratio equals 30%according to the current supply compensation value table 1. Since thebasic compensation value for the energy level 200 MeV is −1.5 Aaccording to the current supply compensation value table 2, thecompensation value equals −0.45 A. Thus, the excitation current value isdetermined as 390 A−0.45 A=389.55 A. In this case, the value in thefourth row of the current supply compensation value table 1 is referredto (since it is the fourth step in terms of the number of steps of theenergy scan) even though 200 MeV is the fifth energy level counted fromthe start (220 MeV) in terms of the energy level.

As above, the excitation current value to be set for each magnet iscalculated by using the current supply control table 1 as the referencefor the excitation current value and the two current supply compensationvalue tables 2 and 3 specifying the compensation current value(compensation value).

In general, the excitation current value of each magnet used fordeflecting the beam increases/decreases with the increase/decrease inthe energy of the beam to be deflected. Therefore, according to thisembodiment, when the accelerator is operated to increase stepwise ordecrease stepwise the energy of the extracted ion beam, the excitationcurrent values of magnets related to the deflection of the beam can alsobe increased or decreased stepwise and their initialization operationcan be omitted as explained above. Consequently, the increase in thetreatment time due to the initialization operation of the magnets can bereduced.

Also for the other bending magnets 15, 20B and 20C of the transportsystems, the excitation current values are calculated by using currentsupply control tables 1 and current supply compensation value tables 2and 3 prepared similarly for these magnets. As a comparative example ofthis embodiment, it is possible to previously calculate the compensationvalues for the excitation currents supplied to the transport systemmagnets for every combination of the extracted energy (energy of theextracted ion beam) and the number of times of changing the energy andprestore the precalculated compensation values in a memory. However, inorder to store all the current value data, the memory is required tostore approximately 2̂N values (N: the number of energy levels). In thecarbon beam therapy, for example, a memory capable of storing 2̂300values (i.e., 10̂90 values) has to be prepared for the total number (300)of energy levels and thus the load on the system becomes extremelyheavy. In contrast, the control in this embodiment calculating thecompensation values from three data tables is capable of reducing theamount of data stored in the memory and lessening the load on thesystem.

Among the control command data (for the bending magnet 20A and the othermagnets) calculated and generated as above, the control command data forthe first energy level are outputted to the accelerator controller 52and the transport system controller 53, while the remaining controlcommand data are temporarily stored in a memory of the central controlunit 50.

After the beam irradiation is started and the irradiation of the deepestlayer (beam scan in the lateral directions) is finished, the settingpatterns for the accelerator controller 52, the transport systemcontroller 53 and the nozzle controller 51 are switched for the beamirradiation at the next energy level. The central control unit 50 readsout the data for the next energy level from its memory, outputs the datato the accelerator controller 52 and the transport system controller 53,and starts the irradiation for the next depth one step shallower thanthe deepest part (in the direction toward the body surface). In theabove example, 404.8 A as the set value for the second energy level isset to the bending magnet 20A of the transport system.

By repeatedly performing such operation, each of the excitation currentvalues for the bending magnets 15, 20A, 20B and 20C arranged along thebeam path in the transport systems is set to change in a shape likeupward stairs or downward stairs. Further, the bending magnets 15, 20A,20B and 20C arranged along the beam path in the transport systems can becontrolled according to settings capable of compensating for thedeviation (change) in the magnetic field (caused by the energy scanoperation) at every energy level.

Furthermore, such operation makes it possible to perform the beamirradiation while maintaining positional accuracy required for theenergy scan operation necessary for the scanning irradiation method.Moreover, since low-priced DC power supplies are used as the powersupplies for the bending magnets 15, 20A, 20B and 20C and appropriateexcitation current values are set for these magnets without the need ofexecuting the pattern operation, treatment irradiation control with highaccuracy of the beam irradiation position becomes possible withoutcausing an increase in the treatment time or the power supply cost.

Incidentally, the same operation method can be employed also for anirradiation method successively changing the energy level in thesynchrotron operation cycle (i.e., extracting the beam at multipleenergy levels by increasing or decreasing the energy level stepwiseduring one operation cycle of the synchrotron including acceleration,extraction and deceleration). Also in this case, beam control realizinghigh beam irradiation position accuracy and reducing the increase in thetreatment time becomes possible. While the present invention has beenapplied to the bending magnets 15, 20A, 20B and 20C arranged along thebeam path in the transport systems in the above embodiment, it is alsopossible to carry out the magnet control by applying an equivalent ideato the synchrotron magnets or to magnets of the transport systems otherthan the bending magnets.

What is claimed is:
 1. A particle therapy system comprising: anaccelerator that accelerates an ion beam to a preset energy level; anirradiation device that irradiates an irradiation target with theaccelerated ion beam; a beam transport system that transports the ionbeam extracted from the accelerator to the irradiation device; and anirradiation control apparatus that controls, in a period in which theaccelerator performs the operation of increasing or decreasing stepwisethe energy level of the extracted ion beam, the excitation current valueof a magnet installed in the accelerator or the beam transport system soas to increase or decrease stepwise the excitation current value basedon the energy level of the extracted ion beam and the number of times ofstepwise changing of the energy level to reach the intended energy levelof the extracted ion beam.
 2. The particle therapy system according toclaim 1, wherein the irradiation control apparatus calculates, based ona compensation value ratio corresponding to the number of times of thestepwise changing of the energy level and a reference magnet excitationcurrent value and a basic compensation value of the magnet correspondingto the energy level of the ion beam, a compensation value for thereference magnet excitation current value and then calculates theexcitation current value of the magnet.
 3. The particle therapy systemaccording to claim 2, wherein: the irradiation control apparatusincludes a first data table in which the reference magnet excitationcurrent values have been recorded, a second data table in which thecompensation value ratio have been recorded, and a third data table inwhich the basic compensation values have been recorded, and theirradiation control apparatus acquires the reference magnet excitationcurrent value of the magnet from the first data table, calculates thecompensation value for the reference magnet excitation current valuebased on the second and third data tables, and then calculates theexcitation current value of the magnet from the reference magnetexcitation current value and the compensation value.